So your asking "After spending $1,500 + on software, why would anyone keep using a $50 dollar app.?" :blink:
I can't think of any reasons.

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Actually, you can just pick up Bryce 5 for free now. I think it's perfect for a person that wants to dip there toes into the 3d world.
Free Bryce + Free D|S is a great entry point in 3D and a brilliant marketting decision by DAZ. When B5 went free, they gained 350,000+ potential customers for nothing, as B5 was no longer even being sold.

Eventually a person may or may not want to invest some more money and pick up something else. They might be content to stay w/ Bryce. There are hundreds of thousands of people who feel that way. If they do decide to upgrade, there's a good chance they will go with Carrara, as it is owned by the same company as Bryce.

Then there is the other end of the spectrum...
After using 3ds max (Maya, XSI, etc.), Vue6 X-Stream (Mojoworld, WorldBuilder, etc.), and ZBrush3 (or Mudbox), why would anyone keep using Carrara?
It just depends what you feel comfortable with and/or can afford.

Friends don't let friends use booleans.